vscode: builder row → expandable phase children, each phase reveals its artifact (spec / plan / diff / review) #792

Problem

The Builders tree shows each builder's current phase in the row label (#773 ... [fix]) but the full phase progression and the artifacts each phase produces are invisible. To see the spec, the user has to know which command opens it; to see the plan, run a different command; to see the current diff, click "View Diff" elsewhere. There's no single tree-driven path that mirrors how a builder actually progresses through a protocol.

Current state

• Server-side already parses the full phase list — parseStatusYaml() populates ParsedStatus.planPhases: PlanPhase[] (packages/codev/src/agent-farm/servers/overview.ts:139) where PlanPhase = { id, title, status } (line 29).
• But BuilderOverview (overview.ts:35) only exposes the current phase string — the parsed phase list is truncated server-side before reaching the client.
• Tree row builder (views/builders.ts:94–98) labels rows with [<current-phase>] only; no expandable phase children.
• Tooltip on the row says Protocol: <p> | Mode: <m> | Progress: <%> (line 108).
• SSE already pushes overview changes; no polling needed.
• codev.viewPlanFile exists for PIR-only plan files (extension.ts:539); the equivalent viewSpecFile / viewReviewFile do not exist yet — see dependency below.

Proposed behavior

1. Flow the parsed phase list to the client

Extend BuilderOverview with planPhases: PlanPhase[]. No new fetch / parser / poll — just stop truncating data that's already parsed.

2. Render each phase as a child row of the builder

Under each builder row, render every phase as a child item with a status marker:

▾ #773 Flaky Dashboard E2E [fix]
   ▸ ✓ investigate
   ▸ ▶ fix
   ▸ ○ verify
   ▸ ○ pr

Icons: ✓ done, ▶ current, ○ pending (use ThemeIcon equivalents).

3. Each phase row is itself expandable to its artifact

Per protocol-specific mapping, expanding a phase row reveals the artifact produced/operated on in that phase:

When a phase has no produced artifact yet (e.g. plan phase pending), expanding shows a placeholder leaf: (no plan file yet).

4. Reactive on phase advance

Tree refreshes via the existing OverviewCache SSE subscription — when porch advances phase or a gate flips, the tree re-renders without polling.

5. The existing "Changed files (3)" child stays

Folds into the Implement / Fix phase row's children (it's the same data), removing duplication.

Dependency

This issue requires P2 (generalize codev.viewPlanFile beyond PIR-only, add sibling codev.viewSpecFile / codev.viewReviewFile). The phase children invoke those commands when clicked. File P2 first or land it as part of this issue's PR.

Acceptance criteria

• BuilderOverview.planPhases: PlanPhase[] ships from Tower (no new fetch — surface the already-parsed data).
• Each builder row in the tree has one child per phase, status icon indicating done/current/pending.
• Phases mapped to a known artifact (Spec/Plan/Implement/Review variants) are themselves expandable to that artifact.
• Implement-phase expansion shows the same per-file list as codev.viewDiff; clicking a file row opens its vscode.diff editor (using existing openBuilderFileDiff).
• Phases without an artifact (investigate, verify, pr, etc.) render as leaves with status markers only.
• Tree updates reactively on SSE overview changes; no polling added.
• Works for all protocols that have parsed phases: SPIR, ASPIR, PIR, AIR, BUGFIX, TICK, MAINTAIN. Protocols without artifact-producing phases render leaf-only.
• The standalone "Changed files (N)" builder child is absorbed into the Implement/Fix phase child to avoid duplication.

Out of scope

• Polling-based phase updates (intentionally — SSE is sufficient).
• Re-architecting how artifacts are stored or named.
• Editing artifacts from the tree (read-only reveal only).
• A separate gates child row (intentionally — gates remain in the current blocked on <gate> row label).